Tim Peake's spacewalk has been stopped after fellow astronaut Tim Kopra discovered water his helmet.

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The spacewalk was terminated by the flight director after Kopra noticed the issue, although Nasa said it was not an emergency. Despite being just a "small bubble" the water was concerning enough to cause an early cancellation of the spacewalk. "The crew is not in any danger," Nasa said.

A tweet form the International Space Station read: "The spacewalkers are safe, taking their time and cleaning up their worksite. @Astro_Tim reported water bubble in his helmet. #spacewalk"

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Both astronauts had already completed their major objective of replacing parts of the side of the International Space Station. The European Space Agency said Peake and Kopra would tidy up their work before heading immediately to the airlock.

It is not the first time water has been found in the helmet of an astronaut: Luca Parmitano, to whom WIRED spoke below, experienced water in his helmet during a spacewalk in 2012.

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Earlier Peake had become the first British citizen to walk in space. The former army helicopter pilot will left the cramped confines the International Space Station (ISS) at approximately 12.48pm GMT, in order to complete some DYI.

Alongside American astronaut Tim Kopra, the Brit will left the space capsule to fix a power supply and lay essential cables. While it is Peake's first foray outside of the space-tin, his mission will last for six hours and 30 minutes, an average time for a walk in space. "It is the opposite of a normal day" astronaut and two-time spacewalker Luca Parmitano told WIRED. "It is a very fatiguing environment. Despite being in very good shape and being very well trained Tim will be tired as it is stressful, and every motion is against a pressurised suit. Every time you move anything you are fighting a force. But of course he is going to be very excited."

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On the night before of the Extravehicular Activity (EVA, in space jargon) Peake published a blogpost saying, in a very British way, that he is looking forward to "popping outside". The astronaut wrote: "Although I am exhilarated by tomorrow’s spacewalk I have no time to dwell on these emotions. The six hours and thirty minutes we will work on the Space Station’s hull are meticulously planned and Tim and I need to execute each step methodically."

Peake's day

Almost all Peake's day is dedicated to the spacewalk. "He's going to wake up hopefully after a good night's rest because he is going to need all the energy and concentration he can get. He's going to have a very hearty breakfast as there is no lunch during the spacewalk," Parmitano said.

After eating, several hours of checks and processes are completed. The station's airlock will be checked, the tools and spacesuit will be configured, but before putting it on medical procedures and a "liquid cooling venting garment" is put on. Parmitano explains: "It is a suit with lots of little tubes all around it and this is used to cool down you body as we need active cooling inside the spacesuit otherwise you would overheat."

Next SpaceX launch will see rockets returning to Nasa's iconic Launch Complex-39A

Light exercises and two hours of breathing pure oxygen will then take place so that the astronauts can tackle the low atmosphere environment and avoid 'the bends'. The spacesuit takes around 20 minutes to put on and then, after suit checks, the pair will move into the airlock, which is depressurised.

The mission

The astronauts will head past the white radiators to the Solar Arrays in the background of this picture.

Nasa

Peake and Kopra have one primary task when they are out in space: repair a failed Solar Shunt Unit. The unit transfers electrical power that is generated by the ISS' solar panels. The station has eight shunts but only seven have been working since November.

The ESA says "the unit is relatively easy to replace" as it is held in place by one bolt. However, once the unit has been replaced the astronauts can't go back inside the station, they then need to lay cables in advance of new docking ports and reinstall a valve that was removed. Their work is heavily planned and coordinated; Peake said it is laid out in a 40-page document.

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Tim Peake preparing for the spacewalk

Tim Peake / ESA

Training

"We have used virtual reality headsets to re-enact our operations and trained for the worst case scenario of becoming detached from the Space Station," Peake said in his spacewalk-eve blog. However, the majority of the training has involved training in swimming pools around Europe simulating every aspect of spacewalks.

"You have to learn how to move around in the spacesuit; how to move your body in a weightless environment; and how to position your body to operate the tools," Parmitano said. The astronauts go through several stages of training underwater, these start with basic communications training and end, hundreds of hours later, with actual mission simulations. "He actually trained on laying the cables that he will be laying on the Space Station. At this time Tim probably has 120 hours underwater inside a station to where he has performed EVAs."

The worst case?

While it is very unlikely that anything will go catastrophically wrong during the mission -- the astronauts are strapped to the space station, after all -- the astronauts have trained for every scenario. Parmitano's second spacewalk was abandoned after two hours when the helmet of his spacesuit started filling with water and he faced the risk of drowning.

Even though they will be hungry, sweaty and tired, the EVA is not quite over yet as they have to clean up the spacesuits and make sure they are in good condition and not damagedLuca Parmitano, ESA astronaut

back inside the space station, if one has to return the other, as after that emergency oxygen supplies will start to run out. The astronauts' spacesuits are heavily equipped with computers and sensors that will tell Nasa, and the spacewalkers themselves, if the suit gets damaged.

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Astronauts are also trained in how to use the rocket-packs on the back of their suits -- for the highly improbable Gravity-style situation of having to fly back to the space station. All crewmembers are also trained in spacewalks to address any emergency failures on board.

The aftermath

"The spacewalk is officially completed after the airlock is closed and they go back to internal pressure. At that point it will take about 12-15 minutes to pressurise the airlock," Parmitano said.

"Even though they will be hungry, sweaty and tired, the EVA is not quite over yet as they have to clean up the space suits and make sure they are in good condition and not damaged."